How is a constant defined in C++?

A constant in C++ is defined as a variable whose value cannot be changed after it has been initialized. This means that once a constant is given a value, that value remains fixed throughout the program, providing a way to ensure that certain values remain unchanged. Constants are useful in programming because they can help prevent accidental modifications to values that should remain static, thus aiding in the readability and maintainability of the code.

In C++, you define a constant using the const keyword, which clearly indicates to anyone reading the code that the value should not be altered. For instance, if you declare const int maxValue = 10;, maxValue will always hold the value 10, and any attempt to change it afterward will result in a compilation error.

The other choices do not accurately represent the concept of a constant in C++. A variable that changes its value does not encapsulate the idea of a constant. A function without a return type describes a different aspect of functions in programming rather than constants. Lastly, the idea of a data type that holds special characters does not relate to constants directly; while constants can indeed hold character data, this definition does not capture the essence of what a constant fundamentally is in the context of programming.